Do you have a Roku TV or a Roku device connected to a TV? If you have a Roku TV, try a "System restart" located under Settings->System->Power. If you have a Roku device connected to a TV, you can try the same thing (although it may be located under just Settings->System), but it's more likely that you've zoomed the display using your TV's remote. Try looking for a button on your TV's remote labeled Zoom, P. Size, or Aspect Ratio.
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